Sharpjiy,

Under the conditions of a strong water drive or gas cap drive, it may not be possible to use the MBE method. In such situations, a small pressure decline takes on a magnitude comparable to the error in pressure measurements, thus bringing too high an uncertainty into the calculations.

Did you attempt a pressure match, to confirm the validity of your model? This is the first thing you should do

Negative OOIP means you may have placed to much emphasis on your early time data and most likely do not have a slope of 1 in your Havlena Odeh plot

See Craft and Hawkins "Applied Petroleum Engineering" Chapter 6

Rgrds

Chee Koh Peh